This invention relates to polyesters and more particularly to an improved process for recycling polyesters. The improved process requires little purification equipment and can handle variable post consumer feedstock or other recyclable polyester materials besides polyethylene terephthalate (PET). More particularly, this invention relates to the recycling of PET, and the production of terephthalic acid (TPA) therefrom.
Polyester resins have widespread utility in a variety of applications, such as films, thermoforming, and blow molding. Additionally, polyesters have widespread acceptance and use in today""s consumer products. Polyesters, such as PET and polyethylene naphthalate (PEN), are used in single and multiple use products such as food and beverage contains, and especially carbonated drink containers, photographic film, magnetic tape, and the like.
In an effort to conserve resources, several methods have been disclosed in the literature for recycling polyesters. Some methods involve depolymerization, i.e., breaking the ester bond and reducing the polymer to its monomer components. Others processes blend virgin polymer with post-consumer materials. These latter processes tend to be simpler and the equipment is less expensive. However, these simpler processes are not without their problems. These processes cannot remove many of the variable constituents, such as colorants and catalyst metals, present in post-consumer polyesters.
Depolymerization of post-consumer polyester into its monomeric components offers more promise since the monomers can in some cases be purified by techniques well known in the art such as distillation, crystallization and filtration. The pure recycle monomers can subsequently be fed to a polyester production process. The cost of the purification steps, however, can make the recycle monomers more expensive than virgin raw materials.
Various methods have been disclosed in the literature for depolymerization of post-consumer polyesters into their component monomers, such as ethylene glycol and terephthalic acid, naphthalene dicarboxylic acid or their derivatives, so they could be reused.
For example, U.S. Pat. No. 3,037,050 discloses the recovery of terephthalate acid dimethyl ester by treating polyethylene terephthalate in the form of bulky or lumpy solid masses with super-heated methanol vapor in the presence of any suitable transesterification catalyst substantially at atmospheric pressure.
U.S. Pat. No. 3,321,510 discloses a process for decomposing polyethylene terephthalate by treating with steam at a temperature of from about 200xc2x0 C. to 450xc2x0 C. The steam-treated polyethylene terephthalate is then reduced from a brittle solid product to a powder having a mean particles size of from about 0.0005 to 0.002 millimeters, after which the fine powder is atomized with a gaseous substance including inert gas and methanol vapor to form an aerosol. The aerosol is conducted through a reaction zone at a temperature of 250xc2x0 C. to 300xc2x0 C. in the presence of excess methanol vapors.
U.S. Pat. No. 3,776,945 discloses a process of depolymerizing polyethylene terephthalate waste to obtain dimethyl terephthalate and ethylene glycol. The waste is subdivided into dimensions between 4 and 35 mesh and treated at a temperature of 100xc2x0 C. to 300xc2x0 C. in the presence of acid catalysts. The proportion of methanol to waste is between 1:1 and 10:1 by weight.
U.S. Pat. No. 4,302,595 discloses an integrated multi-stage process which involves production of a crude ester mixture containing DMT, distillation of the crude ester to produce a crude DMT substantially free of interfering impurities such as terephthalaldehydic acid methyl ester, hydrolysis of the crude DMT in at least two stages to produce TPA and recovery of a purified TPA as well as recovery of methanol.
A crude ester mixture containing DMT is produced by oxidizing p-xylene of technical purity and preferably a mixture of p-xylene and methyl p-toluate with gases containing molecular oxygen in the presence of an oxidation catalyst which contains cobalt and can contain a mixture of cobalt and manganese.
U.S. Pat. No. 4,578,501 discloses a process for the production of fiber grade terephthalic acid from dimethyl terephthalate as the intermediate product by oxidation of p-xylene and/or methyl p-toluate with oxygen-containing gases in the presence of heavymetal-containing oxidation catalysts at elevated temperature and elevated pressure by esterification of the oxidation mixture with methanol at elevated temperature and elevated pressure and by distillatory separation of the crude ester into a methyl p-toluate-rich fraction to be recycled into the oxidation reactor and a residual fraction as well as a crude dimethyl terephthalate having a limited content of oxidation intermediates and other by-products, by continuous hydrolysis of the crude dimethyl terephthalate with water at a mass ratio of crude dimethyl terephthalate to water of between 3:1 and 0.1:1 and at a temperature of between 140xc2x0 C. and 350xc2x0 C. and at a pressure required to maintain the liquid phase to produce a reaction mixture containing crystalline fiber grade terephthalic acid, by recrystallization of the terephthalic acid.
U.S. Pat. No. 4,620,032 teaches an extrusion process for reducing the reaction time in the hydrolysis of polyesters by intimately admixing with the molten polyester a depolymerizing agent which is either one of the products resulting from the complete hydrolytic depolymerization of the polyester or water.
U.S. Pat. No. 5,051,528 discloses a method for recovering ethylene glycol and dimethyl terephthalate (DMT) from polyethylene terephthalate polymers (PET). The process includes the steps of dissolving scrap polyester in oligomers of ethylene glycol and terephthalate acid or dimethyl terephthalate and passing super-heated methanol through the solution. The ethylene glycol and dimethyl terephthalate are subsequently recovered overhead.
U.S. Pat. No. 5,298,530 discloses a process of recovering components from PET having the steps of introducing glycol and terephthalic acid or dimethyl terephthalate oligomers to a first vessel and heating the oligomers, introducing scrap polyesters to the first vessel and forming a start-up melt with the oligomers, transferring the melt from the first vessel to a second vessel, passing super-heated methanol through the melt in the second vessel to form a final melt comprising low molecular weight polyesters in monomers, transferring the final melt from the second vessel to the first vessel, and recovering components in the form of a vapor stream exiting the second vessel. The process shortens the length of the polyester chain in a polyester scrap melt prior to the introduction of the scrap melt to a depolymerization reactor.
U.S. Pat. No. 5,414,022 discloses an improvement to the process of U.S. Pat. No. 5,051,528. The improvement includes the steps of adding polyester to the dissolver and combining it with melt from the reactor and liquid from the rectifier to reduce the chain length of the polyester, transferring the reduced chain length polyester from the dissolver to the reactor, passing super-heated methanol through the reactor to depolymerize polyester into its constituent monomers, transferring the depolymerized products from the reactor to the rectifier, and separating the depolymerized products in the rectifier into a vapor phase containing component monomers and a liquid phase containing higher molecular weight materials.
U.S. Pat. No. 5,576,456 discloses an improvement to the process disclosed in U.S. Pat. No. 5,298,530 in that the reactor for depolymerizing the polyester into its components uses a staged column reactor for the depolymerization and for separating monomer components from the higher boiling materials. The process further utilizes a reactor in which the continuous phase is the super-heated methanol in the molten polyester and polyester decomposition products are the discontinuous phase.
U.S. Pat. No. 5,679,848 discloses a method for making TPA from pure DMT and/or a DMT intermediate product using a counter-current reactor and stripping steam and water under the specific conditions. The DMT is prepared by oxidation of paraxylene and para-toluic acid methyl ester. The method includes the hydrolysis of the DMT in a counter-current reactor and crystallizing the TPA produced without washing it in a subsequent step.
A problem with the all of the above processes for recycling polyester and particularly PET is that the recovered material must be in the form of dimethyl terephthalate (DMT) and ethylene glycol. The reason for this is that DMT, which is reacted with ethylene glycol to form diglycol terephthalate which is then polycondensed to form PET, can be purified using techniques, such as distillation and crystallization, known to those skilled in the art.
Thus, there is a need for a simple and economical method that will permit the recycling of polyesters for recovering suitable feedstock material that may be used in a terephthalic acid (TPA) based polyester manufacturing process.
Briefly, the present invention provides a process for recovering high quality TPA from recycled polyester, and preferably PET, which may thereafter be used in the manufacture of polyester. In one embodiment of the invention the process includes the step of hydrolyzing a DMT containing material derived from recycled polyester to produce TPA.
In another embodiment of the invention, the process includes the steps of depolymerizing the recycled polyester into a product having component monomers, oligomers, half-esters and DMT; substantially segregating the DMT; and hydrolyzing the DMT to form TPA.
In another embodiment, the process includes the steps of contacting the recycled polyester with monomers and oligomers of the same monomers as present in the recycle polyester to produce a solution or melt; contacting the solution with a super-heated lower alkyl alcohol to depolymerize the polyester into a product having monomers, oligomers of the ester, DMT, and half-esters; substantially segregating the DMT; and hydrolyzing the DMT to form TPA and recovering the TPA.
The apparatus used to carry out the process of the present invention is similar to that used in the process described in the copending patent application having U.S. Ser. No. 09/167,248 entitled xe2x80x9cDepolymerization Process for Recycling Polyestersxe2x80x9d filed on Oct. 6, 1998, the disclosure of which is incorporated herein by reference, except that the present invention employs a hydrolysis reactor, and preferably a counter-current reactor where the DMT is introduced at a point that is above the location where water and preferably, steam, is introduced into the reactor. Accordingly, an apparatus for practicing the process of the present invention includes a depolymerization reactor, a separator means for substantially segregating DMT from other materials such as ester monomers, oligomers and half-ester products produced from the depolymerization reactor, and a second reactor operated under hydrolysis conditions for forming TPA from the DMT. It is to be understood that although expressed singularly, it is within the scope of the invention described herein that additional reactors and separators may be used to produce the TPA.
In a preferred embodiment, the apparatus includes a dissolver for at least partially solubilizing or forming a melt of the solid polyester prior to feeding the polyester to the depolymerization reactor.
It has now been found that it is possible to prepare TPA from recycled polyester and particularly PET as compared to the previous methods disclosed in the prior art. This latter step of hydrolyzing the DMT, advantageously and unexpectedly, produces relatively high quality TPA from a recycled polyester material using fewer pieces of expensive distillation and separation equipment.
It is to be understood that the embodiments described herein are for illustrative purposes only and inventive concept is not to be considered limited thereto.